In a folding apparatus which is coupled to one or more rotary printing presses, a plurality of printed webs or ribbons is usually passed over a corresponding number of gathering rollers in order to arrange the ribbons on top of each other before feeding them as a multi-layered web to a pair of nip rollers, arranged on top of a former board for longitudinally folding the layered web.
In prior art folding apparatuses, the nip rollers consist of a driven metal roller, usually a steel roller, and a non-driven rubber or urethane roller which is pressed against the driven metal roller with an adjustable force, in order to adjust the friction between the layered web and the driven metal roller so as to thereby control the tension of the layered web before it passes the former board. Owing to their compact and space efficient design, the gathering rollers and the nip rollers located on top of the former board are arranged such that the layered web is fed at an angle to the nip of the nip rollers. Therefore, the layered web either wraps around both nip rollers or wraps around the circumference of one of the nip rollers, while it only peripherally contacts the other nip roller. Due to the so-called radius effect, the ribbons of the layered web which are passed over the outside of the wrapped nip roller are pulled off more rapidly than the ones which pass over the inside of the roller because of their greater distance from the center of the roller. If the ribbons are afterwards passed to the inside of a subsequent roller, e.g., the nip rollers behind the former board, wrinkles and other undesirable irregularities of the printed and folded products can occur severely affecting the quality of the printed products.
European Patent Application No. 0 747 312 A1 purports to disclose a folding apparatus comprising an arrangement of gathering or take-off rollers for feeding at least two webs as a multi-layered web to an intake roller arranged above a former board for folding the layered web. In this apparatus, the layered web is fed at an angle to the intake roller and the intake roller is formed by a single guide roller which is not in contact with another roller and which therefore does not allow a control of the web tension.
U.S. Pat. No. 5,398,604 discloses a nip roller arrangement comprising a driven transport roller rotatably supported in bearings fixed to a frame and an associated nip roller carrying a removable nip sleeve which is made of resilient material. The nip roller is rotatably supported in pivotable arms and can be brought into contact with the conventionally constructed driven transport roller by means of a pneumatic actuator.
It is an object of the present invention to provide for a roller arrangement in a folding apparatus of a web-fed rotary printing press which eliminates the occurrence of an unequal tension among the respective single ribbons of a layered web when feeding the layered web through a nip formed between a pair of nip rollers.
It is another object of the present invention to eliminate the occurrence of wrinkles and other undesirable irregularities in the printed product which are caused by the radius effect when transporting a layered web through a nip of a pair of nip rollers located on top of a former board for longitudinally folding the layered web.